Will there be detrimental effects if my 450 ohm balanced feed-line comes down the support tree trunk and then runs on the ground for 20-30 feet before entering my shack? In my situation, it is not possible to run the feed-line directly (through the air) from the center of the dipole to the shack window entry point. Tnx. Tom, W4OEQ

The electromagnetic field surrounds a parallel conductor feedline, instead of beingcontained within the shield as is the case with coax. (You can get or make suchfeedline with a shield, but that changes the characteristic impedance and thelosses.)

Running it down the tree trunk isn't too bad - there will be a little additional loss,but probably not enough to worry about. But the ground is a major problem, asit is quite lossy when a significant amount of the field extends into it.

If you can keep the feedline spaced 2 to 5 times the wire spacing away from theground it will improve matters: you may be able to do this with a large diameterdrain pipe and some spacers to keep the feedline centered in the pipe. (Sucha construction could even be buried.) If that isn't practical, you could perhapsdig a ditch and suspend the feedline over the top of it somehow - flush withthe ground, but spaced from the dirt. Another method is to tack it to a woodfence.

You will seriously degrade the system by laying ladderline on the ground. I was experimenting with an antenna a couple of yearsago, and before I got it up into the final position, there was about 20 ft. of the ladderline laying on DRY grass. I figured that dry grass would notmake too much difference. WRONG ! The effect was to "swamp" the system with loss to the point the tuner tuning was very broad. i.e. it was acting like a partial dummy load. The system also showed an odd ball impedance at the input to the ladderline. Once the antenna was up in the air and the ladderline was off the ground, the tuning changed significantly and signal reports improved significantly.

The cold reality is that parallel conductor transmission lines MUST be reasonably kept away from conductive areas/objectsand/or earth ground as a minimum, several times the conductor spacing. That is just the laws of electrical physics; they are what they are. My experience was that laying it on the ground made a LOT of difference (as in "bad").

Remember, the energy field is AROUND that line. If you introduce lossy earth into that field it WILL absorb some of the energy and/orchange the characteristic impedance of the line.

The above said: if you live in a very dry desert area where "earth" is dry sand, it may not be too much of an issue. However,if your soil is moist and conductive, it will make a lot of difference.

If your ONLY possible option is to run line on the ground, I suspect the best bet is to put a good, high quality balun (designed for high SWR/high voltage service) at the base of the tree, then run a very good, high quality, low loss coax into the shack. Can you make small stand off's out of PVC pipe and elevate the line about 12 to 24"off the ground.

Bottomline: laying ladderline or other parallel conductor transmission line on the ground is a bad idea.

Thanks for comments. You all confirmed what I suspected. I have tried "Plan B" in which I used either 300, 450, or 600 ohm feed line to a junction box at the base of the tree and included either a 4:1 or 1:1 balun at that point. I connected high quality coax to the balun and ran the coax across the 20-30 feet and up to enter the shack window (second story of my home). This system worked for most bands but had some "hot spot" frequencies that caused all kinds of problems for the MFJ manual tuner. I understand that others have recommended that the run of coax should be no more than 3 to 6 feet from the window to the equipment inside the shack. This would mean that I would have to run the open wire feeder line directly from the center of the dipole to the shack window (second story of the house). This configuration would not permit me to easily disconnect the feed-line when stormy weather is forecast. I prefer to have a disconnect point at the base of the tree.

I am concluding that the only practical solution is to have a balun at the center of the antenna and run coax to the junction point at the base of the tree. A second run of coax would go to the second story shack window. I would then have to either confine my operation to a single band dipole (40/15 being an exception) or use a fan dipole.

Yes, I can adjust the balanced feed-line to a small degree -- perhaps +/- three feet. If the junction at the base of the tree is to be convenient, it has to be somewhere between my knees and my head. I would not want to create a situation whereby I would need a ladder each time I want to disconnect the antenna. The unanswered question here is, "Why is the length of coax between the balun and the shack an important electrical factor?" I know there is probably a technical explanation, but I do not know the answer. 73, Tom, W4OEQ

Assuming the coax is not in the rf field and acting like a counterpoise, it is not a problem. The balanced line can be somewhat critical in length when it approaches 1/4 wave at the operating frequency. Recall the old days when we used a separate section at the feed point to get our match (Impedance transformation) Some have set up relays to kick in or out different lengths when long hauls are involved. It becomes worh the effort due to having almost no loss on a balanced line compared to coax.RegardsDick KH2G

"Why is the length of coax between the balun and the shack an important electrical factor?"

When a feedline is NOT matched to the load, the impedance varies along the feedline.It's actually rare in modern ham usage for an antenna to be matched to open wire line ortwinlead. (The most common example is a folded dipole fed with 300 ohm line).

If you are feeding a single antenna (such as a dipole) on multiple bands, that means thatnot only does the impedance vary from band to band, but it also varies with the feedlinelength. It is often worthwhile looking for a combination of wire and feedline lengths thatgive a relatively benign impedance on multiple bands (examples are the G5RV or ZS6BKWantennas.) Otherwise we end up with bands where the SWR on the coax section so veryhigh, and much of our RF gets dissipated as heat in the coax before it reaches the antenna.(Like the G5RV when used on 10m.)

We can take a specific example to illustrate the point: let's say we have an 80m doubletand we are using it on 40m with Wireman 551 ladder line. Depending on the wire size,the feedpoint impedance might be 3000 ohms or so. We can use VK1OD's online transmissionline loss calculator to examine the results: http://vk1od.net/calc/tl/tllc.php

With no ladder line, the SWR would be 3000/50 = 60 : 1. That's not too good. If we add20' of ladder line the impedance at the bottom is now 75-j250 ohms, and the SWR has droppedto only 18 : 1. If instead we use 50' of ladder line the impedance is 170+j540, for an SWR of 38 : 1. But a length of 31 feet gives us 55+j1 ohms, so the SWR in 50 ohm coax is 1.1 : 1.

Which of those will have the lowest loss in the coax?

And it gets more complicated because what the tuner in the shack sees is the impedanceat the end of the ladder line transformed through the coax: that initial 3000 ohm impedanceif connected to the 50 ohm coax means the impedance the tuner sees could be 3000 ohms,or 1.2 ohms, or somewhere between those values with some amount of reactance (eitherpositive or negative), depending on the coax length.

That's why it generally works best to arrange for the impedance at the end of the ladder lineto be a low impedance point, as that usually corresponds to minimum SWR on the coax, lowerlosses, and less extreme impedances for the tuner to match. One way to do this on multiplebands is to use a relay to switch extra lengths of ladder line in and out - these can run upand down the tree trunk along with the main feeder (as long as they aren't too closetogether.)

Not that you can't get it to work otherwise, but remember that, just because your tunercan match the impedance presented by the coax, that doesn't mean that your RF isactually making it to the antenna. It isn't difficult to waste 50% or more of your powerheating the coax even when the tuner can find a match.

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